The ATR-Chk1 signaling pathway plays crucial roles in cell cycle control and in the cellular response to replication stress. The mechanistic basis for ATR-Chk1 function is beginning to be understood, thanks largely to intensive study of the pathway in homogenous populations of human tissue culture cells, or simpler organisms such as budding and fission yeasts. Less well understood are the pathway's function(s) in the development and reproduction of animals, and thus to gain a deeper understanding of more specialized functions of this pathway, ATR-Chk1 will be investigated in the metazoan roundworm C. elegans. During C. elegans embryogenesis, ATR-Chk1 controls the timing of cell division in a lineage-specific manner. Preliminary data in support of this application suggest that activation of the ATR-Chk1 pathway during embryogenesis occurs via a different mechanism than has been described in other systems, and thus the goal of Aim 1 is to identify ATR activators and to study the activation mechanism. Aim 2 seeks to understand the basis for lineage-specific signaling of ATR-Chk1 during embryogenesis. Aim 3 examines a novel function for ATR- Chk1, in cell cycle re-entry after long-term arrest in germ line stem cells. During germ line development in C. elegans, the stem cells Z2 and Z3 arrest the cell cycle at prophase, and these cells remain arrested until the larva hatches and begins to feed. Preliminary data in support of this application demonstrate that Chk1 plays a crucial role during the cell cycle re-entry process. Attenuation of Chk1 activity perturbs the timing of reentry, causes DNA damage, and ultimately kills these germ line stem cells, rendering the animal sterile. The goal of Aim 3 is thus to uncover the molecular basis for this novel function of Chk1. Successful completion of the work proposed in this application will significantly broaden our understanding of ATR-Chk1 function during embryonic and germ line development. These experiments will also provide insight into how basic cell cycle processes are integrated with developmental and reproductive events. Finally, insight into ATR-Chk1 function during human stem cell proliferation will likely result from completion of this work, as it is well established that the basic mechanism of asymmetric cell division is highly conserved throughout metazoans. PUBLIC HEALTH RELEVANCE: The ATR-Chk1 signal transduction pathway play a critical role in maintaining genome stability and in preventing cells from accumulating mutations and thereby becoming cancerous. A thorough understanding of ATR-Chk1 function and mechanism is therefore crucial for a more complete understanding of cancer biology. This project will address ATR-Chk1 mechanism and function in a model organism, the round worm C. elegans, with an emphasis placed on novel functions for ATR-Chk1 during embryonic development and in formation of reproductive germ cells.